This invention relates to a drum brake adjuster and more particularly to an improved automatic adjuster for use in an actuator that will have a compact configuration, prevent over-adjustment and which can be simply and conveniently manufactured.
A tappet-type of drum brake adjuster is shown in U.S. Pat. No. 4,380,277, entitled "Automatic Slack Adjusters For Vehicle Shoe-Drum Brakes," issued Apr. 19, 1983 in the names of Brian Ingram et al and assigned to the assignee of this application. This tappet-type of adjuster employs a pair of tappets that are interposed between an actuating member and respective brake shoe ends. The tappets are urged outwardly by the actuator for effecting actuation of the brake shoes. Each tappet comprises two screw-threaded members and an arrangement is incorporated for adjusting the length of the tappets by rotating the screw-threaded members upon wear. In order to equalize adjustment, some form of mechanism is provided for transmitting rotation between the corresponding members of the tappets.
Although the mechanisms shown and described in that patent are extremely effective, the automatic adjusting mechanisms disclosed therein are disposed at the outer periphery of the backing plate and hence increase the size of the brake mechanism at its periphery. This has the effect of limiting the application of this type of brake actuator and adjusting mehanism to brakes having fairly large diameter. However, it is desirable to provide this type of brake in applications where small wheels and drums are required, such as trucks, including those using the new generation of smaller diameter wheels and tubeless tires.
It is, therefore, a principal object of this invention to provide an improved and compact drum brake actuator and automatic adjuster.
It is a further object of this invention to provide a drum brake actuator and automatic adjuster including adjusters of the tappet type wherein the mechanism for adjusting the tappet may be positioned remotely from the tappet.
It is yet another object of this invention to provide an improved mechanism for operating and automatically adjusting a tappet type drum brake actuator.
In the embodiments illustrated in U.S. Pat. No. 4,380,277, the rotation of at least one of the tappet components is accomplished by means of a pinion that engages a gear formation on the outer periphery of the tappet component. Again, this type of construction places the automatic adjusting mechanism at the outer periphery of the brake and incrases the requisite size of the brake assembly resulting in the aforenoted disadvantages. In addition, this type of configuration requires an extended length of the gear formation on the sliding tappet in order to provide sufficient length for adequate sliding support and to accommodate the full travel of the tappet relative to the pinion.
It is, therefore, a further object of this invention to provide an improved automatic adjuster for a tappet type brake actuating mechanism that will reduce the length of gear formation of the individual tappets and still will provide adequate sliding support.
In addition to the size and spatial disadvantages of automatic adjusters and tappet-type actuators as shown in U.S. Pat. No. 4,380,277, the mechanism shown in that patent also will provide adjustment dependent upon the degree of brake shoe travel. If the brake shoes or more specifically the brake shoe ends travel excessively due to a condition other than wear, such as drum deflection, those actuators and adjusters may tend to over-adjust and result in brake drag and excessive brake shoe wear.
It is, therefore, a further object of this invention to provide a drum brake actuator including an automatic adjuster that does not sense drum deflection.
It is a further object of this invention to provide an automatic drum brake adjuster that senses excess clearance only.
In addition to the spatial requirements created by the fact that the interconnecting gearing and adjusting gearing increase the axial length, particularly in order to provide sufficient sliding support area, the arrangement shown in U.S. Pat. No. 4,380,277 places the threaded connection externally of the sliding support and requires a screw-threaded member that extends substantially into the web of the brake shoes. This further increases the size requirements of the assembly and tends to reduce the effective sliding support of the tappet assemblies.
It is, therefore, a still further object of this invention to provide an improved, compact tappet-type brake shoe adjuster wherein both the tappet components of each tappet are slidably supported within the same bore so as to give added sliding support and yet minimize the overall size of the assembly.